Cleaning of products and parts of products to remove contaminants such as waxes, greases, oils and solder flux residues is an integral part of many manufacturing, maintenance, and refurbishing operations. Furthermore, many metallic articles or components parts are treated with oil or other processing agent during the fabrication process, and it is frequently required that this oil must be removed before the article is ready for use or sale or the component is installed in the finished product. Also, excess rosin flux must often be removed from printed circuit boards before the boards are acceptable for use. Solvent cleaning processes are often used to remove contaminants from such articles and parts.
A widely known commercial solvent cleaning process is semi-aqueous cleaning. Semi-aqueous cleaning involves the cleaning of an article in a relatively high boiling point solvent, such as terpenes, esters, or petroleum distillates that have a high affinity for oils, waxes, and greases. Such solvents may be used with or without the aid of a surfactant. Once cleaned of contaminants, the article is commonly rinsed of the high boiling hydrocarbon solvent with multiple rinsing steps using purified water. The hydrocarbon solvent is then typically phase separated from the water and returned back to the wash sump while the aqueous effluent must be processed before sewering to ground water. A drawback of this process is that the hydrocarbon solvent usually possesses a low flash point and, therefore, the solvent must be carefully handled or blanketed with a nonflammable compressed gas such as nitrogen to minimize the risk of explosion. Nitrogen gas is much more fugitive than the dense vapors of a fluorocarbon contained in a condensing zone. Furthermore, in a number of applications, while the article to be cleaned may be compatible with the hydrocarbon solvent, some plastics or metals may be incompatible with the aqueous rinse solvent, resulting in water absorption or rusting of the article.
Chlorofluorocarbons (CFCs), such as 1,1,2-trichloro-1,2,2-trifluoroethane (CFC-113) have been widely used in industry for cleaning and degreasing highly-varied solid surfaces, for example metal components, glasses, plastics, composites and the like. It is common, however, that solvent compositions include cosolvents to improve one or more properties important to the cleaning function relative to the use of any one of the solvents alone. For example, U.S. Pat. No. 6,133,221 discloses cleaning processes that may involve the use certain hydrobrominated compounds together with one or more cosolvents that have a solvent utility in combination with such compounds.
In many prior cleaning applications, there is a concern relating to potential fire or explosion hazards associated with exposing the solvents used in the cleaning composition to a heat source. One approach to limit the potential hazards associated with exposing solvents to a heat source is to utilize only pure nonflammable solvents.
Although the types of solvents that have heretofore been used for the removal of residual contaminants from materials are numerous and varied, many of the preferred solvents are highly flammable. Utilizing only a single solvent may also limit the effectiveness of the cleaning process. For example, chlorinated hydrocarbons, aliphatic hydrocarbons, alcohols, and terpenes are known rosin flux solvents. Because of their low vapor pressures and good solvating ability in the vapor state, chlorinated hydrocarbons, including chlorofluorocarbons, have been widely used in vapor degreasing type processes. However, chlorinated hydrocarbons are generally relatively poor solvents for any ionic residues which may be present on the substrate to be cleaned. For this and other reasons, chlorinated hydrocarbons in general, and chlorofluorocarbons in particular, have sometimes been used in combination with other low-boiling solvents.
An alternative approach to using single component solvents is to use a combination of cosolvents which have no flash or fire point, as disclosed for example in U.S. Pat. No. 6,133,221. This patent discloses that, particularly for degreasing applications, such cosolvents are preferably chosen to provide a cleaning solvent composition which forms either a constant boiling azeotrope or at least a solvent composition which will distill without any substantial change in composition for at least about 24 hours so that it can be recycled without any significant dilution or concentration of any of the components. The use of azeotrope-like compositions which do not exhibit a flash point for drying and cleaning applications is also disclosed in U.S. Pat. No. 6,291,416. The use of non-azeotropic mixtures of components containing flammable solvents has thus heretofore generally been recognized as being dangerous for use in vapor degreasing because flammable compositions may tend to develop in one or more portions of the vapor degreasing apparatus as the cleaning process proceeds.
However, the discovery of non-segregating (azeotropic) nonflammable mixtures of flammable and nonflammable solvents that form azeotropic compositions is difficult and unpredictable. Applicants have therefore recognized the need for methods of cleaning which safely use non-azeotropic solvent compositions to effectively clean articles. Specifically, applicants have recognized the need for non-azeotropic combinations of nonflammable and flammable solvents that segregate during cleaning operations, yet remain nonflammable throughout the cleaning operation.